How to "ifdown ..." on squeeze?

On a new installation of squeeze, ifdown no longer works as it used
to (on my old Debian system).
When ifconfig lists an interface "eth0", neither "ifdown eth0" nor
"ifdown eth" takes the interface down. The attempts yield:
"ifdown: interface eth0 not configured" and
"ifdown: interface eth not configured".
How does one take an interface down on squeeze?
(My old system's /etc/network/interfaces had:
...
auto eth0
iface eth0 inet dhcp
On my squeeze system, it's:
...
allow-hotplug eth0
(with a commented-out line:
#NetworkManager#iface eth0 inet dhcp
)
)
Also: The reason I was trying to take it down and back up was to
trigger a fresh DHCP query and corresponding host-name configuration.
What's the preferred way of doing that?
Thanks,
Daniel